6" of water is not very deep for a skimmer area. bigger skimmers usually require at least 8-9" of water to work effectively. you can get by with 6", but it'll put a lot of stress on the pump and could lower the logevity of your skimmer. and with a 75 gal, you would want to get a skimmer rated for 150g at least. and those bigger skimmer usually require higher level of water to run at optimal performance. If you haven't bought it, I wouldn't suggest buying this for your tank. If you have, i would do research and find a big skimmer that has a lower water level recommendation, but that might be hard to find unless you go external. Also, If you want to go with a DSB in your refugium, you won't be able to with this refugium as the holes/cut out in the baffles are very low, looks only like 3". NOT to say it wouldn't work. but i think there are better options out there for the price, such as a custom made sump.
